The Main Factors Causing the Crack of Sendzimir Roll
The crack of the Sendzimir roll occurs at the transition point from the Sendzimir work roll body to the upper roll neck, within 40-100mm from the upper end of the Sendzimir work roll body. Most of the roll neck cracks are transverse cracks, with two forms of ring cracks and local cracks. If the crack is light, it can be found during machining. If the crack depth is less than the machining allowance, it can be removed during machining. If there is still a crack after machining, it shall be scrapped. If the crack is heavy, a circle of ring cracks can be seen in the as cast state, which will cause the roll neck to be broken in serious cases.
(1) Chemical composition. In the production of rolls with cupola, the melting sequence is arranged from low to high according to the roll name (SI): cold hard roll, ball core roll, ball compound roll, an infinite cold hard roll, and nodular iron roll. In order to ensure that the temperature of the first ladle of the 10t cupola is higher than 1400 ℃, the height of the bottom coke is specified as 2.8m. Due to the high coke content in the bottom of the cupola, the increment of WC and WS in the front several packages of molten iron is relatively serious, which results in that the WC of the alloy chilled cast iron roll is as high as 3.75% - 3.80%, the WC is as high as 0.115% - 0.120%, sometimes even as high as 0.132%.
(2) Pouring temperature. When using cupola to produce roll, the temperature of the first and the second ladle is generally low. In addition, the temperature of the front ladle and the ladle is low and the heat absorption is large, which results in the low pouring temperature of the molten iron, the poor fluidity of the molten iron and the early solidification of the riser, which destroys the directional solidification and increases the tendency of cracks.
(3) The deeper the white layer of the roll, the greater the scrap amount of the crack of the Sendzimir roll. This is because the contraction of white mouth is larger than that of hemp mouth and grey mouth.
(4) The inner wall of the riser sand mold is uneven, which hinders the free downward contraction of the roller neck.
(5) The greater the ratio of the length (L) of the work roll to the diameter (d) of the work roll, the higher the rejection rate of the crack. For example, the scrap rate of cracks in the rolls with a diameter of 355mm × l020mm and L / D = 2.87 is as high as 50%. The greater the ratio of L / D, that is, the longer the length of the work roll, the greater the shrinkage of the length direction of the work roll, and the higher the probability of the upper roll neck crack.
(6) If the length of the work roll body of Sendzimir is more than 700mm and the diameter of the work roll body of Sendzimir is less than φ 350mm, the roller neck sand core is adopted, the sizing block is not removed after pouring, or the outer circle of the sand core is too large, and it is too tight with the cold mold, so the effect of reducing the shrinkage resistance of the roller neck sand core (it can move down after pouring) is lost.
(7) The crack of Sendzimir roller is related to the characteristics of pig iron. In the production process, it is often found that when pig iron is exchanged, the roll is prone to crack (including longitudinal crack of the work roll body, edge crack of upper and lower rolls, neck crack, etc.), while after another pig iron is exchanged, all kinds of cracks will be reduced or disappear. It may be due to the influence of the heredity of the cast-iron structure, which makes the roll shrink and increase, so it is easy to cause cracks.
(8) When the amount of tellurium is too high, the crack tendency of Sendzimir roller increases. Adding tellurium to cold hard roll can increase the depth of pure white mouth, shorten the length of hemp mouth, and improve the wear resistance of roll and the surface quality of steel. In order to obtain a pure white layer, 8-12g tellurium is added to each ton of molten iron, which is higher than that of domestic counterparts, so the scrap rate of roll neck crack is higher than that of domestic counterparts. Tellurium is a strong anti graphitization element. Because of the high content of tellurium in the molten iron, the amount of graphite precipitated in the solidification process is reduced, and the tendency of cracks is increased.
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